I get the synth as close as possible using MIDI volume and panning automation and then freeze it. Because further automation after the freeze is likely, there'll always be a volume envelope (and sometimes a pan envelope) added to the frozen audio track.
Where this method falls down is when I need to do some creative panning as part of the mix, and it's a stereo synth track. In that situation, I'll either drag the frozen audio into a new track and insert Channel Tools for pan automation, or freeze the track without freezing the fx bin. The latter method lets you unload the synth so it's not using any resources while retaining all your options for effects, including stereo panning.
For very large projects with many synths (more than a dozen or so) I'll freeze each synth track, gather them up into a track folder, and then ctl-drag each one into a new track. The original frozen tracks are then archived and now I've got an all-audio project ready for mixing. Even my aged machine can easily handle a hundred-track project when it's all audio.